Japanese Earthquake Highlights Importance of GNSS Timing

One of the more surprising consequences from the March 2011 earthquake in Japan was that the forces unleashed shifted the earth's mass sufficiently to accelerate its rotation, shortening each day by no less than 1.8 microseconds, according to calculations by NASA. And while such effects are clearly imperceptible to humans, their cumulative effect on GNSS timekeeping would cause significant inaccuracies that would be unacceptable to any GNSS application.

Each GNSS will deal with the time variation in its own way, generally by transmitting a correction factor to each satellite in the constellation, which will then be embedded in the navigation message received by each GNSS receiver. This therefore emphasises the importance of ensuring that any GNSS receiver design has the ability to cope with time corrections sent from its constellation.

Fortunately, Spirent's SimGEN™ software allows users to modify any part of the navigation message simulated by the company's GNSS simulators. This includes the ability to simulate timebase changes that might be required to compensate for unusual phenomena such as the effects of the Japanese earthquake. This type of test is more usually applied to ascertain the receiver's ability to recognise the insertion of leap seconds.

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